In recent years, it has been studied actively to produce semiconductor devices, e.g., a blue LED, a white LED, and a violet semiconductor laser, by using group 13 nitrides, e.g., gallium nitride, and apply the resulting semiconductor devices to various electronic apparatuses. Conventional gallium nitride-based semiconductor devices are produced mainly by a vapor phase method. Concretely, production is performed through heteroepitaxial growth of a gallium nitride thin film on a sapphire substrate or a silicon carbide substrate by, for example, a metal organic vapor phase epitaxy method (MOVPE). In this case, the thermal expansion coefficients and the lattice constants are significantly different between the substrate and the gallium nitride thin film and, therefore, high density dislocations (one type of lattice defects in a crystal) occurs in gallium nitride. Consequently, regarding the vapor phase method, it has been difficult to obtain high-quality gallium nitride having a low dislocation density. Meanwhile, besides the vapor phase method, a liquid phase method has also been developed. A flux method is one of the liquid phase method. In the case of gallium nitride, by using metal sodium as a flux, the temperature and the pressure, which are required for crystal growth of gallium nitride, can be alleviated to about 800° C. and several MPa to several hundred MPa, respectively. Concretely, a nitrogen gas is dissolved into a mixed melt of metal sodium and metal gallium, and gallium nitride becomes supersaturated, so that a crystal grows. As for such a liquid phase method, dislocations do not occur easily as compared with the vapor phase method. Therefore, high-quality gallium nitride having a low dislocation density can be obtained.
Research and development on such a flux method have also been performed actively. For example, Patent Document 1 discloses a method for manufacturing a group 13 nitride crystal for the purpose of reducing the dislocation density. Concretely, in the flux method, a substrate, in which the normal to the principal surface has an inclination angle of 1° or more and 10° or less relative to the <0001> direction of a gallium nitride seed crystal layer, is used as a base substrate containing the gallium nitride seed crystal layer, and during growth of a gallium nitride single crystal on the principal surface of the base substrate, dislocations remaining in the gallium nitride single crystal are propagated in a direction parallel to the {0001} face so as to be discharged to a peripheral portion of the single crystal.
By the way, in FIG. 2 of Patent Document 1, as for the principal surface of the base substrate, a stepwise uneven surface having a plurality of microsteps is exemplified. These microsteps are formed from a plurality of terrace surfaces, which are {0001} faces (c-faces), and a plurality of step surfaces, which are a plurality of {1-100} faces (m-faces), and adjacent terrace surfaces are connected to each other with the step surfaces therebetween. Here, crystal growth in a direction parallel to the terrace surface is predominant to crystal growth in a direction perpendicular to the terrace surface and dislocations are propagated in the direction of crystal growth. Therefore, the dislocations generated during crystal growth are propagated substantially parallel to the terrace surface so as to be discharged to a peripheral portion of the crystal. In this regard, it is explained that a substrate, in which the normal to the principal surface has an inclination angle of 1° or more and 10° or less relative to the <0001> direction of a gallium nitride seed crystal layer, is used as the base substrate and the inclination angle exceeding 10° is not preferable because microsteps are generated during crystal growth, and the melt is entangled into the microsteps to generate vacuoles (inclusions). In general, a macrostep refers to a step having a height difference on the order of micrometers which can be observed visually or with a microscope of low power easily, and a microstep refers to a step, e.g., an atomic step, having a height difference on the order of nanometers.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-51686